Poland is the biggest loser in Eastern Europe. After Kievan Rus was crushed by the Mongols, Poland was by far the strongest East European state remaining. Moreover, in the 14th century Poland united with the Grand Duchy of Lithuania to form a new superstate which in English is called "the Commonwealth" (don't remember how it's spelled in Polish, but it's their version of Latin "Res Publica"). Compared to them, the Grand Duchy of Moscow was very weak, both economically and in terms of the population. However, the Grand Duchy of Moscow (which would become Russia around the time of Ivan IV) had a moral drive to liberate all Russian lands from foreign occupation. It was mostly Russia that stripped Poland from all its non-Polish colonies and left it as a marginal state that it is today, as compared to the almost superpower status it enjoyed in the 15th century. And that's considering that Russia was one of the weakest European states until well into the 17th century.
So, Poles will never forgive Russians for that. Not a big loss, though.
